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The Tube Family Tree, Part 1May 1963 Popular Electronics

May 1963 Popular Electronics

People old and young enjoy waxing nostalgic about and learning some of the history of early electronics. Popular
Electronics was published from October 1954 through April 1985. All copyrights are hereby acknowledged. See all articles from
Popular Electronics.

For some inexplicable reason I went backwards on this three-part
Tube Family Tree series that appeared in Popular Electronics.
Author Louis Garner, Jr., starts out with the early history of vacuum
tubes, beginning with Thomas Edison's incandescent light bulb and
then quickly progresses to Lee de Forest's Audion amplifier tube,
and on through the evolution of multi-grid vacuum tubes that are
specially designed for low noise receiver front ends, high power
transmitters, voltage and current regulators, video cameras, pulse
forming networks, traveling wave tubes, and many other types. There
is quite a bit of information and history contained in these three
installments that will do a very nice job of introducing you to
the wonder that is the electronic vacuum tube.

The Tube Family Tree, Part 1

By
Louis E. Garner, Jr.

Today, there are easier ways of handling electrons. Yet, the
vacuum tube was, is, and will remain one of the 20th century's greatest
inventions ...

There are some who say that the electron tube has been doomed
by the transistor and its semiconductor "cousins," and that the
tube, as the dinosaur, will become extinct. To these statements,
the tube, if it could speak, might well reply in the words of the
proverbial old man - I ain't dead yet. Actually, the tube is very
much alive and kicking, with new types being introduced on an almost
day-to-day basis, and handling more different kinds of jobs than
ever before.

The tube "family tree" is a vigorous, strong, and healthy growing
plant. Let's examine it closely, starting with the roots and exploring
the many branches and twigs.

Birth of the Tube. The electron tube had its
beginnings in distinguished company. Thomas Alva Edison, one of
the greatest inventors of all time, was experimenting with his newly
invented incandescent lamp bulbs one day when he discovered a curious
phenomenon. When he installed a small metal plate in his lamp bulb
near the glowing filament, and connected this plate through a sensitive
galvanometer to the positive filament connection, he found that
a small, but easily measured, current flow took place. Unable to
explain the reason for this at the time, he nevertheless realized
that it might be potentially significant, so he obtained a patent
in 1883 on what came to be called the "Edison effect."

For many years, the Edison effect remained a classroom curiosity
without any commercial value. Edison had, however, unknowingly invented
the first true electron tube - the elementary two-electrode type
we now call a diode.

In
the meantime, scientists in other fields had started a chain of
events which, eventually, would have a profound influence on the
application of the barely understood Edison effect. In 1887, Heinrich
Hertz had demonstrated that electromagnetic waves operate in accordance
with the laws governing light and heat waves, and described the
basic theory upon which modern radio communication is based. Just
after the turn of the century, J. A. Fleming, searching for an improved
detector for electromagnetic waves, found that the Edison effect
could be used advantageously. It was Fleming, then, who invented
the first practical diode - the Fleming valve. The name "valve"
stuck, incidentally, and, even today, electron tubes are called
"valves" in most parts of the world outside of North America.

The next great step forward came when Lee de Forest added a grid-like
wire structure between the filament and plate of the diode, patenting
his new device, which he dubbed the Audion, in 1906. It was de Forest
who gave the electrical valve (or electron tube, as you prefer)
an entirely new capability - the ability to amplify as well as detect
weak electrical signals. He invented the triode tube, and, in so
doing, laid the basic foundation for our great radio, television,
and electronics industries.

Three Basic Jobs. The electron tube's operation
is relatively simple, once the idea of electron emission is accepted.
Free electrons are liberated (or emitted) by the glowing hot filament.
Since these elementary particles are negatively charged, they are
attracted to the positively charged plate, moving across the intervening
vacuum to it. This current flow is unilateral - that is, from filament
to plate and not vice versa. It is this property which permitted
the early Fleming valve to act as a detector. The filament, since
it served as a source of electrons, came to be called the cathode.
Later, this term was applied generally to any electron source in
a tube (or other device), whether a filament or not.

It all started with Edison's observation that electric
current flowed back to the battery in this experimental
light bulb.

Over 20 years later, de Forest found he could control
current flow by placing a grid-like structure between plate
and filament.

When de Forest added his grid-like wire between the filament
and plate of the Fleming valve, he found that a small voltage applied
to this structure could influence the plate current. Again, the
operation is relatively simple. A negative voltage applied to the
grid (as it came to be called, in deference to its original appearance)
repelled the negative electrons and reduced plate current; a positive
voltage applied to the grid attracted additional electrons and accelerated
them towards the plate, increasing plate current. Since the grid,
an open-like structure, intercepted few - if any - of the electrons
moving towards the plate, its power requirements were very minute
- but it could control a relatively large plate current. Thus, a
small voltage applied to the grid was able to control plate current
and hence amplify signals.

The ability to amplify made another feat possible. Once a device
can be used to amplify, a small part of the amplified signal can
be fed back to the unit's input (grid). The device then serves as
its own source of signal, and this feedback can generate alternating
currents - so, it becomes an oscillator.

With the addition of the grid, the electron tube became capable
of performing the three basic jobs it has handled from the first
decade of this century to the present day: detection, amplification,
and oscillation.

The
Tube's Evolution. Fleming's valve was a two-element tube,
or diode, and consisted of a filamentary cathode and a plate. If,
as was discovered, the filament is operated on 60-cycle a.c. line
power instead of d.c., a certain amount of the line "hum" will appear
in the plate circuit. This led to the addition of a separate cathode
... essentially a metal tube coated with chemical elements which
emit electrons when heated.

The tube is still a diode, however, with the indirectly heated
cathode and the plate serving as its principle elements. The filament
is reduced to the simple role of heating the cathode and, quite
appropriately, is often called a heater.

When
de Forest added a control grid, the tube became a three-element
device, or triode. In practice, the triode was found to suffer from
a serious disadvantage when used as a tuned r.f. amplifier. There
is a fair amount of electrical capacity between the grid and plate.
This permitted a percentage of the signal in the plate circuit to
be coupled back to the grid, setting up the basic condition for
oscillation.

Thus, early r.f. amplifiers tended to be quite unstable, and
a variety of schemes were developed to neutralize the effects of
grid/plate capacitance. In general, these consisted of introducing
enough inverse (or negative) feedback from the plate tuned circuit
into the grid circuit to counteract the direct feedback inside the
tube and prevent unwanted oscillation.

The next forward step came with the introduction of a second grid
to serve as a shield or screen between the (control) grid and plate.
By reducing grid/plate capacity, the four-element tube, or tetrode,
permitted r.f. amplifiers to be assembled without special neutralization
circuits.

It was soon discovered, however, that the tetrode, as the triode
before it, had a peculiar disadvantage of its own. For effective
shielding, the screen grid was given a positive charge. This accelerated
the electrons tremendously in their trip to the plate, causing the
electrons to strike the plate with sufficient force to "knock" other
electrons off the plate material. In a sense, the plate became an
electron emitter, secondary to the cathode.

With the proper combination of plate and screen voltages, more
electrons would be emitted by the plate than were received by it,
and these, traveling back to the positive-charged screen grid, caused
a curious phenomenon. Under certain conditions, an increase in plate
voltage would cause a decrease in plate current . . . as if the
tube acted like a negative resistance. Again, the net result was
instability and the tendency to oscillate when the tetrode was used
as an amplifier.

In
an effort to reduce, or "suppress," the plate's secondary emission,
a third grid was added between the screen grid and plate and connected
back to the tube's cathode, creating the five-element or pentode
tube. This third grid was called, appropriately, the suppressor
grid. It served to repel the secondary electrons back to the plate
without appreciably affecting the normal cathode-to-plate electron
flow. Today, the pentode is perhaps the most widely used basic tube
type.

Somewhat later, it was found that a single cathode could be used
for more than one function if additional electrodes were added outside
the normal control range of the grid elements. This, in turn, led
to the development of multi-purpose tubes.

To make greater use of the stream of electrons moving
from cathode to plate, this system of beam confining was
developed. Practical all modern power amplifier types use
these beam-forming plates.

Beam Power Tube. The basic screen grid, whether
in a tetrode or pentode, requires a fair amount of power for operation-power
which does not, however, contribute to the strength of the amplified
signal. Where very small amounts of power are handled, as in weak-signal
amplifiers, the power loss is small and relatively unimportant.
If a large amount of power is handled, as in tubes designed to handle
several watts, then the power loss becomes significant, contributing
to a loss in operating efficiency.

This led to the development of the beam power tube in which the
control and screen grid wires were aligned in the same plane, so
that the resulting flow of electrons was formed in parallel sheets
or "beams" between the cathode and plate. Beam-confining electrodes
were added to shape the resulting beams and confine the electron
flow between the grid wires, thus preventing electron movement to
the support leads on which the grids were wound.

The resulting tube had much smaller screen currents than earlier
tetrodes (and pentodes) with comparable power-handling ability,
and thus was more efficient. At the same time, it was found that
the beam power tube had much greater power sensitivity than earlier
types. Today, beam power tubes are used extensively at both r.f.
and audio frequencies where more than a few watts are involved.

Gas-Filled Tubes. The majority of electron tubes
are designed to operate within a vacuum, so that there will be no
gas molecules present to interfere with the free movement of electrons
between the cathode and plate electrodes. In fact, during the manufacturing
process, a metallic substance is evaporated within the tube, forming
a film on its walls and absorbing the last traces of gas: this element
is called the getter.

For some applications, however, limited amounts of specific gases
may be introduced in a tube. Where a gas is present, the gas atoms
can, under certain conditions, ionize; the gas atoms are partially
stripped of their outer electrons and become positively charged
ions. The gas ions move towards the cathode while the free electrons
move towards the plate.

Gas
may be ionized by the application of heat and moderate voltages
or by the application of relatively high voltages. This latter fact
has led to the development of several types of cold-cathode tubes
- tubes which do not require a filament or heater - the simplest
examples of which are diode rectifiers such as the OZ4, voltage
regulator tubes, and neon lamps.

The presence of gas within a tube has several primary effects.
First, of course, positively charged gas ions tend to reduce the
tube's effective plate-to-cathode resistance, thus reducing its
internal voltage drop. For this reason, gas-filled (generally, mercury-vapor)
rectifiers are extremely popular where large currents are handled.
Second, an ionized gas gives the tube an "all or nothing" characteristic.
Until ionization occurs, relatively little current can flow. Once
the gas is ionized, however, current reaches a maximum very quickly,
and stays at that maximum value (determined by load resistances
and supply voltages) until the plate voltage is reduced to a very
low value or cut off altogether.

Gas-filled triodes, or thyratrons, like gas-filled diodes, have
an "all-or-nothing" characteristic, but with one difference. The
difference is that the control grid in the thyratron, being relatively
close to the cathode compared to the plate, can be used to "trigger"
ionization even though the plate voltage is below the value normally
required to ionize the gas. Thus, a small signal (or trigger) voltage
applied to the grid can switch the tube from a non-conducting state
very quickly. Thyratrons are used extensively as relaxation oscillators
and for control and switching applications.

A number of gas-filled cold-cathode triodes have been manufactured
for special applications. Their operation is somewhat similar to
that of the conventional thyratron, except that no filament is used.
The most familiar example of this type of tube is the flash tube
used in photographic electronic flash lamps.

Receiving Tubes. By far the most popular general
class of tubes are low- to medium-power types primarily designed
for use in radio and television receivers. This general class encompasses
all the basic tube types-diodes, triodes, tetrodes, pentodes, beam
power, and multi-purpose tubes.

In the early days, there was little need to identify tubes except
by their manufacturer's name, for most units were essentially the
same. Later, as more types were developed, identifying type numbers
were introduced. These served to identify a particular type of tube
in terms of its characteristics, permitting tubes produced by different
firms but having the same type number to be used interchangeably.
The first type numbers were simple numerical designations, such
as 01A, 15, 19, 20, 42, 45, 76, and 80.

As more and more types were developed, a different numbering
system became necessary. The tube manufacturers decided to adopt
a system of numbers and letters such that the type number itself
would give an indication of the tube's basic application. With this
system, the first number would indicate the tube's nominal filament
voltage, a middle letter the intended application (amplifier or
rectifier, for example), and the last number the number of active
elements. Amplifier type tubes were to receive letter designations
from the first part of the alphabet, rectifiers from the end of
the alphabet.

Multiple triode sections in one glass envelope are not
as new as you might think. The 6A3 tube of the 1930's (right)
had two triode sections, but they were wired together. The
current compactron (left) has three triodes in its envelope,
and each grid, plate, and cathode goes to a separate base
pin.

Gone but not forgotten are these three pioneers. The
200A (left) was one of the first mass-produced tubes. A
954 Acorn helped introduce VHF to communication services.
At right is an 1851, a super-high-gain tube developed just
before World War II.

As the internal structure of vacuum tubes became more
complex, the necessity arose to provide pin connections.
Here are some examples of the changing styles from 1925
to 1963. Along the back row (left to right); 4-pin and 5-pin
tubes, circa 1925-1935; and the octal, 1935-19-. In the
front row: the loctal (left). and the popular 9-pin miniature
tube.

How Numbering System Worked. A type 6A3 was
a tube with a nominal 6-volt filament (actually 6.3 volts), an amplifier
type (A), with three active elements - filamentary cathode, grid,
and plate. The 2A3 was a similar type, but with a 2-volt (actually
2.5-volt) filament. Similarly, the 6D6 was a tube with a 6-volt
filament, an amplifier (D) type, with six active elements - filament,
cathode, control grid, screen grid, suppressor grid, and plate.
A 5Y3 was a tube with a 5-volt filament, rectifier type (Y), with
three elements - filamentary cathode and two plates; the 5Z4 was
similar, but with an extra element - an indirectly heated cathode.

Unfortunately, even this seemingly ideal system was inadequate,
for the introduction of new types soon outran available type numbers.
Today, type number designations still use number-letter combinations,
and the first number generally (not always) indicates nominal filament
voltage, but the remaining part of the type number does not always
hold to its original meaning.

Tube Envelopes. Tube construction techniques,
too, have undergone many changes as the "family tree" has grown.
Originally, tubes were assembled in glass envelopes almost identical
to those used for incandescent lamp bulbs. Later, refined designs
were introduced with connection pins which allowed tubes to be plugged
into their sockets (rather than screwed in, as with lamp bulbs).

Metal envelopes became popular because they carried the advantages
of built-in shielding and were less prone to breakage than glass
types. Special shapes were designed for high-frequency tubes to
reduce electrode leads to minimum length ... such as the now-famous
Acorn tube. Still later, miniature 7- and 9-pin glass types were
developed.

Today, the miniature glass tube is the most popular, general
type, although several new types of construction have been introduced
in recent years.

Pentagrid Converter. As radio receiver circuits
became more and more complex, multi-purpose tube designs became
increasingly popular. The use of these permitted more compact receiver
chassis layouts without, at the same time, compromising circuit
sophistication. Some of these multi-purpose units were (and are)
simply the elements of two or more tubes combined in a single envelope,
but some special types were developed, the most popular of which
is the pentagrid converter.

The
pentagrid converter has, as the name implies, five grid structures.
Its primary application is in superheterodyne receivers, where it
is used as a combination local oscillator and mixer.

In operation, the cathode, control grid, and screen grid are
used as the essential elements of a "triode" tube with appropriate
components to form an oscillator. The incoming r.f. signal is applied
to the second control grid (grid 2), which is shielded by the two-element
screen grid. The cathode-to-plate electron stream is common to both
control grids, hence the locally generated signal and incoming r.f.
signal are combined in the stream and electrically "mixed" by the
time the stream reaches the plate.

The Compactron. Developed primarily for TV applications
and representing the present-day "ultimate" in multi-purpose tubes,
the compactron is a squat, 12-pin tube with a glass envelope similar
to that employed with 7- and 9-pin miniatures (but broader) and
may combine the functions of as many as three or four different
tubes in a single unit.

Ceramic Tubes. An ultra-miniature type designed
for high-frequency applications, ceramic tubes are made up "sandwich-fashion"
with alternate metal electrodes and ceramic spacer-insulators. They
are used primarily in VHF and UHF receiver designs, in radar equipment,
and in similar applications, although a few types have been designed
for TV receiver and FM set work.

The Nuvistor. Just as the compactron represents
the present-day ultimate in multi-purpose tube construction, the
nuvistor is the latest version of the metal envelope tube popular
a few years ago. Nuvistors are manufactured in the basic generic
types (triodes, tetrodes, etc.) and are extremely small physically
... not appreciably larger, in fact, than the typical transistor
and actually smaller than some power transistor types. They are
especially well suited to compact receiver design and are used extensively
in TV, FM, and VHF receivers.

Part 2 of this article will explore the transmitting tube "branch"
of the tube "family tree," as well as the development of various
types of phototubes and cathode-ray tubes.